WASHINGTON — The United States slapped sanctions on Syria’s foreign minister and two other top officials on Tuesday as it pressed President Bashar al-Assad’s regime to stop its crackdown on protesters.

The sanctions imposed by the Treasury Department are the third in a series of punitive U.S. measures placed on Assad and top government officials and follow calls by U.S. President Barack Obama for Assad to step down.

State Department accuses Muallem of playing key role in trying to insulate regime from implications of its own brutality.

Asset freezes and bans on business interactions were imposed on Foreign Minister Walid Muallem, top presidential advisor Bouthaina Shaaban, and Syrian ambassador to Lebanon Ali Abdul Karim Ali, the Treasury Department said.

“Building on our sanctions targeting the entire government of Syria, we are bringing additional pressure to bear today directly on three senior Assad regime officials who are principal defenders of the regime’s activities,” said Treasury Under Secretary David Cohen. State Department spokeswoman Victoria Nuland linked the sanctions to “the role that (the three) play in propagating and advancing the reign of terror that Assad is exacting on his own people,” who are demonstrating for democracy.

She charged that Muallem has “continued to beat this drum of international conspiracy and has attempted to cover up the regime’s horrific activities by making claims that terrorists or others were responsible.”